Information stored at a point on a magneto-optical storage surface is generally detected by analyzing the polarization of a beam of light that has been reflected off the point on the surface. A detector head receives the reflected light beam for analysis. The angle of polarization of the light beam rotates upon reflection in a manner that is dependent upon the magnetic field present at the point of reflection on the storage surface. Information is stored on the surface in the form of magnetic fields oriented in different directions. Differences in the resulting angle of polarization following reflection indicate the state of information stored on the surface at the point of reflection, since the direction of angular rotation of the polarity is determined by the direction of the magnetic field. The magnitude of the angular differences corresponding to different magnetic fields is typically small, on the order of plus or minus half a degree. Variations in the amplitude of the reflected light beam, however, are often as large as plus or minus ten percent of the total magnitude. Given the relatively large amount of amplitude fluctuation, accurate detection of polarization angles is generally difficult.
One way to determine variations in polarization angle is to split the reflected light beam into two orthogonally polarized component light beams, A and B. This can be done with a polarizing beam splitter. The beam splitter is oriented at a 45.degree. angle from the polarization angle of the incoming beam. In the absence of any rotation of polarization, the A and B component light beams are of equal magnitude. With the beam splitter oriented this way, any rotation of the polarization angle results in one component having a larger magnitude than the other. The direction of the polarization angle rotation determines which of the components has the larger magnitude.
The A and B components can be detected by separate photodetectors, each generating electrical signals based on the magnitude of either the A or B polarization component of the reflected light beam. The direction of angular rotation, and consequently the direction of the magnetic field at a reflection point on the surface, can be determined by subtracting the A and B components, with the desired information appearing in the sign of the difference signal. Fluctuations in amplitude, which should appear equally in both the A and B components, should not affect the sign of the result of the subtraction. This is known as common mode rejection.
In reality, because the A and B components follow different paths from the beam splitter to the module which implements the differencing, and these paths have slightly different optical and electrical properties, the A and B components each undergo a slightly different variation. These unequal variations can introduce a bias into the difference signal, so that the result of differencing the signals is inaccurate. Because the polarization angle rotations to be detected are very small, even slight inaccuracies introduced from the paths of the signals can result in the polarization angle information being masked by the errors, making it difficult or impossible to extract the desired information.
In order to minimize the inaccuracies in the difference signal, compensation of either the A or B signal is necessary to account for the optical and electrical path differences. This is conventionally done by varying a potentiometer setting which adjusts a gain for one or both signals. The potentiometer is set so that, when no polarization angle rotation is present in the reflected light beam, the result of the differencing operation is as close to zero as possible.
Such a solution to the problem of individual path variation is generally not sufficient where a magneto-optical drive uses multiple detector heads with the differencing of signals being performed in a single module. A magneto-optical storage device can use more than one detector head to accommodate multiple storage surfaces. Because the paths for the A and B components corresponding to each detector head are different, a separate gain adjustment is necessary for each detector head. It would be impractical, however, to adjust a potentiometer on the differencing module each time a different detector head is selected. What is needed for such a magneto-optical storage device is a mechanism for automatically compensating for the individual path variations.